Cpu that wont bottleneck a 4870

I'd go for the PhenomII X3 720 over the 7750, in fact I'd choose that over any other similar priced CPU.

But Q9550 is twice the price of the P2 720.

But the P2 720 is a tri core you'll have to compare the P2 940 and the Q9550 and that is where I'm coming from.

Please ignore the Intel fanboys. They think everyone should run a q9550 or i7 920, despite them costing twice to thrice as much as what is necessary. Yes, you would notice the difference between a Athlon 7750 and a Phenom 2 720. First, the Kuma is a first-generation Phenom core, which is cache-starved, and runs at a lower hypertransport. Not to mention it is a dual-core. The Ph2 is triple-core, and so will run core-optimized games better, as the third core can be used to run background aps and the OS. It has a faster HT link, more cache, more OC headroom, and if you're lucky, you can enable the fourth core and get a 940 cheap.

What I'm saying it that in the original post he said he wants a cheap system and since he was thinking of a P2 720 one can assume that's roughly as far as his budget will go. You may aswell suggest a QX9770 while you're at it

Absolutely!
The difference is huge!
Phenom II 720 is great choice for the money, it speed is compared to old phenom 9950! and most important is that it's clockable to 3.6Ghz - 3.9Ghz !!!